Hypersonic jets in fast lane

Peter Pae, Los Angeles TimesCHICAGO TRIBUNE

For more than four decades, scientists have been trying to develop a jet-powered aircraft that could zip across the sky at five times the speed of sound, a feat that has been harder to accomplish than sending a man to the moon.

But in a hangar in the desolate flats of the Mojave Desert, a team of engineers is putting the final touches on a new plane they believe could shatter the enduring barrier and lead to an airliner flying from Los Angeles to New York in 30 minutes.

It has been 54 years since a plane broke the sound barrier--675 m.p.h. Next month a 12-foot experimental airplane is scheduled to make its maiden flight, soaring over the Pacific Ocean near Los Angeles at more than 5,000 m.p.h.

Although the first flight of the unmanned X-43A is expected to last about 10 seconds, covering about 14 miles, it would mark a dramatic turning point for the long effort to develop a hypersonic aircraft, a vehicle that is powered not by a rocket but by an air-breathing jet engine.

"We're finally getting down to testing the basic science of a new propulsion system that could ultimately alter commercial aerospace and national security," said Charles Vick, acting director of space policy for the Federation of American Scientists.

It would come after repeated setbacks in the bid to design a hypersonic craft. Now, with one short flight, NASA officials say dreams of a commercial airliner that can fly from Los Angeles to Tokyo in two hours instead of 10 will be one step closer to reality. The military's vision calls for a bomber too fast to shoot down.

NASA officials said the test flight is expected to take place May 19 at the Dryden Flight Research Center at Edwards Air Force Base, where the plane is being prepared. The plane, dubbed Hyper-X, was designed by Boeing Co. at its Seal Beach, Calif., operation and built by Micro Craft Corp. in Ontario and Tullahoma, Tenn.

"No one has ever flown an engine like this at the velocity we are talking about." said Vincent Rausch, NASA's Hyper-X program manager.

For scientists, the flight will represent a milestone in the development of propulsion systems and aerodynamic designs. The plane, resembling a boogie board with twin tail fins, will be propelled by a supersonic combustion ramjet engine, or "scramjet," essentially a rectangular copper box mounted to the belly.

But looks are deceiving, because the science behind the scramjet has been one of the more difficult technological barriers to overcome.

The common turbojet uses turbines inside the engine to compress air, which ignites with kerosene to create combustion and then thrust. Although the turbojet is efficient at subsonic speeds of conventional airliners, air flows too slowly and the engine overheats at high speeds.

Ramjets are basically a hollow tube with no moving parts, resolving the temperature problem. They can propel an aircraft past Mach 2, or twice the speed of sound. Airflow into the front of the ramjet is compressed and mixed with fuel. Their combustion creates thrust as it exits the engine.

But the ramjet cannot power an aircraft past Mach 5. That requires a scramjet, in which gases can flow at supersonic speeds. Although it is mechanically simple, it is vastly more complex aerodynamically than a jet engine.

Interdependent design

For instance, the front end of the X-43A, such as the flat nose, helps compress the oxygen before it enters the copper alloy chamber, where it mixes with hydrogen and burns, creating pressure from the expanding gas to propel the plane forward. In essence, the propulsion system and the airframe are interdependent, a radical new concept in aircraft design, NASA officials said.

"It's a subtle difference, but you have to think of the plane and the engine as one," said Joel Sitz, NASA's project manager for the X-43A at Dryden. "The plane is integrated into the engine, and that makes it much more efficient."

But for a scramjet to kick in, the aircraft must be flying fast to suck in enough oxygen for combustion to occur. As such, the X-43A will be mounted to an Orbital Science Corp. Pegasus rocket, which boosts the aircraft to the required speed, or about Mach 5 or greater.

Once detached from the booster rocket, the aircraft will be able to fly between 7 and 10 times the speed of sound, or about 4,725 m.p.h. to 6,750 m.p.h., depending on altitude and atmospheric conditions. Most commercial jets cruise at about 500 m.p.h., and the world's fastest plane, the Air Force's SR-71 Blackbird, flies at about 2,100 m.p.h.

For the test flight, a modified B-52 bomber with the X-43A attached to the wing will fly to the California coast and launch the aircraft along with the booster rocket near San Nicolas Island, southwest of Los Angeles.

The X-43A will fall away from the B-52 as it is cruising westward at about 20,000 feet. About 5 seconds later, after it is clear of the carrier aircraft, the booster rocket will fire, lifting the X-43A to about 100,000 feet, where the hypersonic plane will separate and fly on its own for about 10 to 15 seconds, covering 14 miles or so before plunging into the ocean. By comparison, the Wright Flyer flew for 12 seconds and covered about 120 feet when it made aviation history in 1903.

If all goes according to plan, the X-43A should reach a speed of Mach 7, or about 5,000 m.p.h., breaking the record for both air-breathing and rocket-powered airplanes, surpassing the Mach 6 record set by the rocket-powered X-15 in 1967.

Goal of Mach 10

Two additional X-43A planes are under assembly for flights planned within the next year, concluding with the third aircraft attempting to reach Mach 10. NASA officials said there are no plans to recover the vehicles, which will be used primarily to gather data and test the propulsion system for the next generation of hypersonic planes, dubbed X-43B and X-43C.

Eventually, NASA hopes to develop a plane that could take off on its own, use rocket power to reach Mach 5 so that the scramjet could kick in and then use rockets again to propel it into space.

One advantage of the scramjet is that it does not require bulky and heavy oxygen tanks to power the rockets, freeing up cargo space. NASA officials said the hypersonic plane eventually could replace the Space Shuttle.

Before passenger-carrying hypersonic planes are developed, engineers see more cargo and military uses. FedEx Corp. and United Parcel Service Inc., two of the world's largest freight companies, are intrigued by the prospect of ferrying packages at such high speeds, NASA officials said.

The total cost of the X-43A program--$185 million--is a far cry from the more than $2.5 billion spent on an earlier hypersonic aircraft effort, known as the National Aero-Space Plane.

The program was kicked off by President Ronald Reagan less than a week after the Space Shuttle Challenger disaster in 1986. He described an "Orient Express" that by 2000 could "take off from Dulles Airport, accelerate up to 25 times the speed of sound ... flying to Tokyo within two hours."

But with a recession and Congress under pressure from critics who argued that such a plane would not fly, funding dwindled and prompted several aerospace companies to abandon or significantly scale down the project.

NASA kept up the research, however, tapping into studies completed for the National Aero-Space Plane. The project left a legacy of about 20 trailers filled with invaluable research papers and data, NASA officials said.